One application of polyolefin-based adhesives is for multilayer films. Multilayer films are widely used in the packaging industry. Depending on the intended end-use of the film, the number and arrangement of layers as well as the types of polymer resins employed will vary. Polymer resins used include a wide variety of polar and non-polar polymers selected depending on their ability to provide essential properties such as structural integrity, puncture resistance, abrasion resistance, etc. Resins having barrier properties are generally included as a barrier layer to prevent the passage of oxygen and moisture in multilayer constructions used for food packaging. Commonly used resins for barrier layers include polyethylene (PE), ethylene-vinyl acetate (EVA) copolymers, ethylene-vinyl alcohol (EVOH) copolymers, nylons and the like. To facilitate adhesion of layers of dissimilar polymer resins, for instance between a PE layer and an EVOH or nylon layer, adhesive layers referred to as tie layers are employed. The tie layers are generally mixtures of functionalized polyolefins and non-functionalized polyolefins.
Another application of polyolefin-based adhesives is for polymer-metal multilayer pipes. The multilayer pipes are increasingly used for plumbing and heating applications. PE and PEX (cross-linked PE) are commonly used polyolefins; aluminum and copper are commonly used metals. The multilayer pipes usually have the structure of PEX/tie/metal/tie/PEX or PE/tie/metal/tie/PE. Although the metal provides most of the mechanical strength in these constructions, mechanical integrity and pressure ratings of the multilayer pipes depend on the quality of the tie layer between the metal and PE.
Maleated polyolefins are commonly used functionalized polyolefins in tie layer adhesives. Maleating a polyolefin is usually performed by visbreaking, with or without free radical initiator, the polyolefin in the presence of maleic anhydride. The maleated polyolefin is typically let down with non-functionalized polyolefin resins to a desired concentration and to provide a polyolefin-based adhesive. See U.S. Pat. No. 4,487,885. The maleated polyolefin can be let down by a variety of polyolefin resins, including high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), polypropylene and ethylene-vinyl acetate copolymers. High performance adhesives also utilize a polyolefin elastomer as a let-down resin.
While many polyolefin-based adhesives are known, new adhesive compositions are always in demand to meet the requirements of new polyolefin laminate applications. Particularly, there is a need for polyolefin-based adhesives which have improved adhesion in multilayer polyolefin films and pipes.